1. Field of the Invention
The invention relates generally to a dielectric ceramic material applicable to the fabrication of dielectric resonator components for microwave and millimeter wave frequency bands. More specifically, it relates to the spinel (MgAl2O4)-system dielectric ceramic composition having a high quality factor and a low dielectric constant, and a method of fabricating dielectric ceramics using the same.
2. Description of the Prior Art
As a demand on comprehensive information communication service including personal mobile communication service has been increased, the development of communication technology and system capable of rapidly transmitting a large quantity and a various kind of data are keenly required. In order to realize the high-speed wideband information communication service of the next-generation, it is necessary that the frequency resources of a millimeter wave capable of providing the wideband characteristic should be used. Therefore, there have been many actively running researches on communication systems and components that can be used at the millimeter wave frequency band.
It is required that the components for the millimeter wave frequency band should have extremely low loss characteristics, since the millimeter wave is attenuated severely when it propagates through the air and RF circuits. The components with low loss characteristics are possibly manufactured by using the low dielectric loss materials along with the optimization of the structure design. Generally, the dielectric materials for the communication components should have a large dielectric constant and also a low dielectric loss value for the purpose of miniaturizing the components"" size and low insertion loss, respectively, although there are many cases that these two requirements conflict each other. Because the wavelength of the millimeter wave is very short, however, the demand for a large dielectric constant becomes less important, while the demand for extremely low dielectric loss becomes more important. Therefore, low dielectric loss materials should be used in manufacturing components that utilize the dielectric resonator characteristics such as filters or duplexers at the millimeter wave band. In addition it is also recommended that those materials have a very small (or approaching to 0) temperature coefficient of resonant frequency (xcfx84f).
Dielectric ceramic compositions developed so far for the microwave applications have the dielectric constant exceeding approximately 20, and the value of the quality factor (Qxc3x97f) of them is below about 100,000, which is the barometer of the dielectric loss. However due to the large propagation loss of the millimeter wave the quality factor of the dielectric materials is required to be as large as over 100,000. And also, considering the short wavelength of the millimeter wave, it is required that those materials should have the dielectric constant below 10. Alumina (Al2O3) is a candidate material satisfying above conditions of the dielectric characteristics. This material, however, has usually been used only for a substrate material, and dielectric ceramic compositions with a low loss value in the microwave/millimeter wave band have rarely been developed so far.
The present invention is contrived to solve the above problems, and the object of the present invention is to provide a dielectric ceramic composition with a low dielectric loss value that can be used in the microwave/millimeter wave frequency band, and also the manufacturing method of the dielectric ceramics. The present invention employs spinel (MgAl2O4) as a basic composition, and a small amount of lithium carbonate (Li2CO3) as a sintering aid is added to MgAl2O4 in order to improve the sintering and consequently dielectric characteristics.
The spinel (MgAl2O4)-system dielectric ceramic composition of the present invention can be characterized in that the spinel (MgAl2O4) is used as a major component and lithium carbonate (Li2CO3) is used as a sub component, wherein the composition formula of them is as follows;
Composition formula:
MgAl2O4+xLi2CO3(mol %) 
where, 0 less than xxe2x89xa65 (mol %)
Also, a method of manufacturing the dielectric ceramic composition according to the present invention is characterized in that it comprises the step of mixing of magnesia (MgO) and alumina (Al2O3) as raw starting materials with the mole ratio of 1:1, the step of calcining the mixture, and the step of adding lithium carbonate (Li2CO3) of 0.1xcx9c5 mol % in excess to the calcined mixture, remixing, shaping and sintering sequentially;
Composition formula:
MgAl2O4+xLi2CO3(mol %) 
where, 0 less than xxe2x89xa65 (mol %)
Preferably, the calcination process is performed at the temperature of 1300xcx9c1350xc2x0 C. for 4 hours and the sintering process is at 1600xcx9c1650xc2x0 C. for 4 hours.